A good soil test is one which is simple, rapid and can remove all or a representative portion of the available nutrient pool in a wide variety of soil types. Few existing tests excel in both these requirements. Methodology which provides the best estimate of nutrient availability is often too complicated and cumbersome for routine labs making fertilizer recommendations to producers. Furthermore, many existing P, N, S and K tests do not take into account all factors affecting nutrient availability in soil. Some tests are specific to a region or soil type, performing poorly when transposed to other environments. For example, P and K tests which are based on chemical removal of specific P and K fractions are usually limited in geographic scope since the importance of a fraction may vary depending on the physical and chemical environment of the soil.
Anion-exchange resins are considered one of the better indices of plant available P. Cation-exchange resins have also been used for extraction of available K in soils. However, conventional resins in bead form are generally difficult to separate from soil and are not well-suited for routine analysis. Still, there are numerous prior art references teaching the use of ion-exchange resin beads to evaluate nutrient availability in soil samples.
In Brazilian Patent 8605998, nutrients are extracted from soil samples for analysis using capsules with at least two opposite fabric walls containing standard amounts of ion-exchange resins which are placed in a beaker with a sealed cover. Measured amounts of dry soil and a fixed amount of water are added in the beaker, placed on a special tray and agitated in a specially built machine by free tumbling. Beakers are taken to a washer-separator and washed with deionized water to remove soil and other particles. The capsules are then transferred to a clean beaker containing a solution of NaCl and HCl. The resulting solution contains the substances to be analyzed and the resin is regenerated by washing with successive solutions to remove cations and anions and restore its exchange capacity.
A technique of this type has at least two major drawbacks. Firstly, there is the necessity of bringing a soil sample to the laboratory and to dry such soil sample, a step which can require up to three days. Secondly, the use of resin beads is extremely impractical as it is very difficult to wash from the beads the soil that could have accumulated during agitation of the capsules containing the resin beads in the soil suspension. Also the bags are susceptible to fraying and rupture due to abrasion by the soil during shaking.
U.S. Pat. No. 4,816,161 relates to an ion extractor comprising a tube filled with cation or anion-exchange resin beads. It is used for extracting ions from streams, lakes and marine sediments. The resin beads are retained in a dialysis tube by support tubes which each have a screen of an appropriately-sized mesh attached to the interior ends by means of a suitable adhesive. Hence, when it is used, the extractor is placed in an aqueous suspension of the soil sample to be analyzed. Once the resin has been contacted with the soil suspension, it is eluted with acid in order to leach the exchange ion from the exchange sites and to return the resin to a homoionically saturated exchange state for subsequent reuse. The sample leachate is collected in tubes and analyzed to determine the concentrations of constituents of interest. Again, in this type of system, it is required to produce a soil suspension as direct contact between the soil and the ion-exchange resin beads is avoided.
U.S. Pat. No. 4,775,513 and re-issue 33,487 relate to a device for water treatment that uses a water-tight container of flexible material containing ion-exchange material. The ion-exchange material can be selected from silicates, clays and synthetic resin beads. The invention uses colorimetry to determine the exhaustion of the ion-exchange capacity of the resin beads. The container is filled with water and shaken to allow contact between the beads and the ions.
It is suggested to use the device described in U.S. Pat. No. 4,775,513 as a soil-testing device. However, the device is impractical as the chemicals to be tested for must first be extracted from the soil and it is necessary to filter the soil extract prior to submitting it to chemical analysis.
In Sibbesen (1977, Plant in Soil, 46:665-669), a method is described, whereby ion-exchange resin beads are sewn in nylon-netting bags which are used to extract available phosphate from a soil-water suspension.
Similarly, in Skogley et al. (1990, Communications in Soil Sciences and Plant Analysis, 21:1229-1243), the use of ion-exchange beads sewn-up in bags is reported as a test for P, K and S availability. In this procedure, soil samples are brought to the lab, water is added and a saturated paste is prepared by addition of water to the soil until it is completely saturated. As the authors note, saturated pastes are difficult to prepare uniformly and reproducibly. As well, the authors note that the test was only successful for K and S, amounts extracted were very small and difficult to measure and the extraction time required is 2 days or more. Added to the fact that the method is cumbersome and requires considerable amount of time, a special vacuum extractor instrument is required in the elution step.
In Yang et al. (1991, Soil Sci. Soc. Am. Journal, 55:1358-1365), the authors provide theoretical considerations of the testing approach outlined by Skogley et al. However, nothing is suggested to modify or improve the Skogley et al. technology.
In Saggar et al. (1990, Fertilizer Research, 24:173-180), the authors describe a simplified procedure for determining the amount of phosphate extracted from soils by using ion-exchange resin membranes in soil suspensions. Again, the procedure presents some of the drawbacks described previously.
In a paper entitled "Universal bioavailability of environment soil test" (International Symposium on Soil Testing and Plant Analysis, Aug. 22-27, 1991, Orlando, Fla.), E. O. Skogley describes research work in which anion and cation-exchange resins contained in nylon or polyester bags were buried in the face of soil pits for 6 months to study nutrient movement after an intense forest burn. As mentioned previously, there are problems in desorbing or stripping the nutrient ions of the resin beads contained in the bags. Furthermore, the bags do not work efficiently in the field as muddy soil debris often penetrate through the netting. Once inside the netting, the soil is very difficult to wash out. The washing step is extremely important as if the beads are not washed properly, the acid that is used in the elution will dissolve all of the P, S and N in the soil, not just the plant available ions on the resin, giving erroneous results. Another drawback of this type of method is the fact that soil debris are often found in the final eluent which interferes with the final analysis. Also, one of the reasons why very few papers have been published on the actual burial of resin beads in soil stems from the fact that the time period required to extract measurable amounts of nutrients from soil using beads ranges from at least 1 to 5 days and amounts to unrealistic values.
The potential of anion and cation-exchange membranes has been evaluated for routine soil-testing in laboratory environments. Essentially, the method consists of immersing the ion-exchange membrane in a water-soil suspension, washing the immersed membrane with water and diluting the membrane in an acidic solution to displace the ions immobilized on the membrane into the solution. The concentration of ions displaced in the acidic solution is then determined using standard analytical methods. This type of method is also in many respects unsatisfactory and does not solve the major drawbacks encountered with previous techniques. That is, that it is a laboratory measurement requiring that soil samples be brought to the lab and that many important soil characteristics are destroyed during drying and handling.
The laboratory-technique requires that soil samples be brought to the laboratory and dried prior to preparation of a suitable soil suspension. This procedure is cumbersome and expensive as large storage space is required for the soil samples. As well, there is the risk of contamination of the soil samples during transport and when they are laid out to dry. In plant nutrient testing assays, the time turnaround is critical for both the farmers and the fertilizer companies that depend on soil-testing facilities to evaluate the mount of fertilizer required in a particular soil. Furthermore, in a laboratory environment where soil suspensions are tested for nutrients, the only manner in which ions are drawn onto the membranes is by diffusion through the shaking of the membranes within the suspensions. The test is conducted based on a suspension that usually does not take into account the biological availability of nutrients based on the natural soil environment.
Other techniques not using ion-exchange resins or membranes for soil testing have also been described in the art. None of them has been successful at providing accurate nutrient availability index.
U.S. Pat. No. 4,201,549 describes a soil-testing technique that uses dialytic tubes. Two soil samples are placed in separate containers, each containing a dialysis unit. One dialysis tube of each unit contains lithium carbonate and the other tube contains acetic acid. The dialysis units are removed from the containers after 24 hours. The soil is washed away from the units and the solutions are analyzed.
This method suffers from numerous drawbacks. Firstly, the time required to transfer the ions from the soil onto the dialysis membranes seems to be about 24 hours, which is impractical to carry rapid on-site determination of available nutrients in soil. Furthermore, the technique does not solve the problem of taking soil samples to the laboratory as two separate soil samples must be removed from the soil. Also, the samples must be placed in separate isolated containers, which adds another cumbersome step to the method. Another problem is the fact that dialysis tubes are fragile and would not appear to be suitable to withstand repeat insertion and removal in soil samples.
U.S. Pat. No. 4,126,417 describes a method for determining the presence of a limited number of soil nutrients. The method uses paper strips treated with a pH-testing coating and a nitrate-testing coating. Typically, the strips are very insensitive and give a crude approximation of nitrogen availability in soil. Furthermore, the strips can only be used for determining pH and nitrate amounts, the other elements being washed away prior to testing.
The determination of residual profile nitrate was widely used as the criteria for fertilizer N and S recommendations in Western Canada and USA because limited leaching occurred before planting. However, organic N mineralization during the growing season by microbial processes can provide a substantial amount of inorganic N as NH.sub.4 --N and NO.sub.3 --N and improved the degree of prediction of N fertilization needs. More accurate N fertilizer recommendations could be obtained if actual contributions from mineralization in the farmers own field could be indexed, which may differ considerably from the "average" for an area. At present, this cannot be done efficiently.
Leaf tissue analysis for nitrogen (N) and phosphorus (P) is used to provide an estimate of the current N and P status of plants at the time of sampling. Various methods, with different plant parts, are used for tissue analysis for N and P. Total nutrient concentration in the tissue may be used as the diagnostic criteria, or else just a fraction such as water extractable ion. The method by which total N concentration is determined has been most widely used in the past. However, one of its major drawbacks resides in the fact that the method does not allow one to selectively measure nutrients in the plant and is complex. A good method of leaf tissue analysis should be simple and sensitive. Potassium removed by simple HCl solution and sulfate by water extractions of fresh plant tissue have shown to be good indexes of potassium and sulfur deficiencies, but require a time consuming + error prone filtration step.
The quick testing of heterogeneous liquid samples also poses some inconvenience as the presence of bulky impurities tend to affect spectroscopic measurements. In order to circumvent this problem, the heterogenous sample has to be purified prior to testing. Very often, the purification procedure is complicated and requires the use of sophisticated and expensive purification separation equipment.
In a document entitled "The determination of trace amounts of cobalt and metals in high purity water by using ion-exchange membranes" (Analyst, April 1973, Vol. 98, p. 274-288), H. James describes a method by which porous cation and anion-exchange membranes are used to analyze the presence of ions in the cooling water of nuclear reactors. The porous membranes are enclosed inside an apparatus connected to a flowline on the reactor and water continuously passes through the membranes. The author mentions that this technique has some efficiency but that it must be used within a sophisticated filtration apparatus. Hence, it was found to be essential to incorporate prefilters immediately before the ion-exchange membrane in order to prevent blockage of the membrane as water is passed through it.
In situations where it is desired to investigate the presence of trace chemicals in a particular sample such as a soil sample, a sediment sample or a water sample, it is usually required that the analyzed trace chemical be first extracted from the sample concentrated, and then detected using various spectroscopic techniques.
In conclusion, there are at present no available tests for rapidly conducting on-site determination of unbound ions found in a liquid or solid medium, or for determining the amount of free unbound ions in living cells. A simple and convenient test could find particularly useful applications in the area of soil nutrient testing.